docs(plugins): reconcile the hook reference with HookType (missing 9 hooks + 2 categories) #1743

The hook reference in docs/docs/concepts/plugins.mdx presents itself as exhaustive — "Plugins are organized into six hook categories" and "Enum with all 18 hook types" — but has drifted out of sync with the HookType enum (mellea/plugins/types.py), which now defines 27 hook types across 8 groups. So the doc both omits a third of the hooks and makes a false completeness claim.

Missing from the doc (9 hooks, 2 whole categories):

• Generation: generation_batch_pre_call, generation_batch_post_call, generation_batch_error, generation_event — the doc lists only 3 of the 7 generation hooks.
• Adapter functions: adapter_function_invocation_complete, adapter_function_phase_complete — category entirely absent.
• Streaming: streaming_start, streaming_event, streaming_end — category entirely absent.

Why it matters: beyond the stale list, the "all 18" claim actively misleads. For example, sampling_loop_start is documented as firing "when a sampling strategy begins execution" with loop_budget writable — so a reader using stream(strategy=..., loop_budget=N) could reasonably expect it to fire and let them tune the budget, but streaming never fires the sampling_loop_* hooks (it uses the streaming_* hooks + RetryEvent). This surfaced in the #1697 review.

Fix — pick one direction:

1. Exhaustive and kept so: document the 9 missing hooks + the Adapter and Streaming categories, correct the counts (6 → 8 categories, 18 → 27 hooks), and add a guard (a test or doc-gen step) that keeps the reference in sync with HookType so it can't silently drift again.
2. Representative, not exhaustive: drop the completeness claim, keep a curated set of common hooks as examples, and link to the authoritative full list (the HookType enum / API reference). Lower maintenance; won't re-drift.

Scope: one doc file (docs/docs/concepts/plugins.mdx) plus HookType for the inventory; no runtime code. Good first issue — well-bounded, and a nice way to learn the hook surface.